Single compressor systems are well known for cooling. Such systems are commonly used in refrigerated containers for trucks, rail and shipboard transportation of food products. Note however, that the present invention is not restricted to transportation applications of refrigerated container.
Single compressor containers are unequal to certain cooling tasks. For example, once produce is picked it is desirable to immediately reduce its temperature to prevent spoiling. During hot summer months, shippers of produce transport the produce to a cold storage warehouse to bring the produce down to temperature before loading onto a refrigerated container which merely operates to maintain the temperature of the precooled produce are incapable of bringing the produce to adequate temperature quickly enough. The ability of such transportation devices to cool a hot load of produce in a sufficiently short time to prevent spoilage does not exist in commercially-available containers.
Most commercially-available refrigerated containers for transportation are single compressor systems. Generally, single compressor systems are inadequate for cooling a load below about -20.degree. F. Some commercially-available refrigerated containers having single compressor systems can cool a load to about 0.degree. F. Unfortunately, purchasers of refrigerated containers desire a device which can maintain a load at -20.degree. F. and lower at ambient temperatures up to +150.degree. F.
By way of example, consider a single compressor system for cooling a load to -20.degree. F. in an ambient environment of +150.degree. F. The evaporator temperature necessary to maintain the load at a predetermined temperature is at the best 10.degree. F. colder than the load. Here the evaporator is cooled to -30.degree. F. Under these conditions using R12, the evaporator pressure is expected to be approximately 9 psi and using R22, the expected pressure is approximately 20 psi. Similarly, the condenser temperature necessary to discharge heat to the ambient is 10.degree. to 40.degree. warmer than the ambient under the best case conditions; thus, in this example, the condenser is at 160.degree. F. The pressure in the condenser under these conditions is expected to be approximately 278 psi for R12 and 445 psi for R22.
The conditions in the example of the previous paragraph dictate a compression ratio of 278/9.apprxeq.31 for R12 and 445/20.apprxeq.22 for R22. Refrigeration compressors are designed and built to operate with a compression ratio no greater than 15. If the pressure ratio exceeds the manufacturer's design criteria the compressor will break. Accordingly, neither example above could be achieved with a conventional single compressor system. Indeed, a commercially available compressor is not available with the capacity to operate in a refrigerated container environment under the above conditions and accordingly, such a system in a refrigerated container would be prohibitively expensive and inefficient. Thus, commercially available single compressor systems are incapable of operating where the difference between the desired product temperature and the actual ambient temperature is very large as in these examples.
Cascade systems are well known. It is well understood in cascade systems that heat from a lower cascade condenser is removed by the evaporator of a high cascade compressor system; and heat from the high cascade system is dissipated into the ambient. The pressure ratio for the cascade system is the product of the pressure ratio for both the low cascade compressor system and the high cascade compressor system. A cascade system for the R22 example described above would also have a pressure ratio of approximately 22 and it could have both the low and high compressor systems operating at the same pressure ratios, i.e., both pressure ratios at approximately 4.7 for each compressor. This pressure ratio is well within an acceptable range of the specifications of commercially available compressors.
Cooling systems require a minimum pressure ratio to operate. If the necessary pressure ratio becomes too small the compressor will fail. As the difference between the product temperature and the ambient temperature is reduced, the pressure ratio for a cooling system is also reduced. When using a cascade cooling system, as the difference between these temperatures becomes smaller, the pressure ratio for both compressor systems will fall below the minimum pressure ratio necessary for operation sooner than a system using a single compressor.
Conventional cascade systems use different refrigerants, one for each compressor in each system. This requires the system designer to uniquely design the low compressor system and the high compressor system. Commonly used refrigerants are 502 and R12. To protect the environment, these refrigerants will be banned after 1995. The refrigerant R22 is far less damaging to the environment than 502 or R12 and as such is not scheduled to be banned until 2020.
What is needed is a cooling system for cooling a load of product to a desired temperature which can efficiently operate over a broad range of ambient temperatures, e.g., -60.degree. F. to +150.degree. F. and load temperatures from -25.degree. F. to +75.degree. F.